Crossflow vs. Looper

[randlemanboater]

OK, who will explain the difference in a crossflow and a looper.

And what do you think that a 1986 or 87 V-4 seadrive will be?

Which is better?

[phester]

skools is the OMC man on this site ;)

[Skools Out]

86 crossflow most likely but 87 Looper Small bore

Post pic of power head.

[Skools Out]

http://www.wellcraftv20.com/cgi-bin/yabb/YaBB.cgi?board=Performance;action=display;num=1134 098488;start=0#0

[Skools Out]

TWO STROKE DEFINED
The two-stroke engine completes its power cycle in only one crankshaft revolution with two strokes of the piston. There are no valves, camshafts, springs chains, etc. so the engine is much less complex and lighter. Instead of valves There are a series of strategically located transfer ports - intake and exhaust, cut into the sides of the cylinder wall. The ports are on opposite sides of the cylinder. The transfer ports are opened and closed by the up and down movement of the piston. To accomplish a complete power cycle both sides of the piston are used; consequently several events occur simultaneously during each stroke. They are:

Up Stroke - Intake and Compression:
On the up stroke the top side of the piston is compressing an air/fuel mixture in the cylinder. At the same time the BOTTOM side of the piston pulls another fresh charge of air/fuel mixture into the crankcase thru a one way valve called a reed valve. Near the top of the stroke the compressed air/fuel above the piston is ignited by the spark plug and begins to burn. The rapidly burning fuel expands and begins forcing the piston down.

Down Stroke - Power and Exhaust
On the down"power"stroke the piston is forced towards the crankcase reducing its volume and creating a positive pressure. As it continues downward travel it starts first to uncover the exhaust ports. Exhaust gas begins to rush out of the cylinder. Then the intake ports are uncovered. The fresh air/fuel charge in the crankcase is forced into the cylinder and continues to push the remaining exhaust gases out.

The 2 stroke process of purging exhaust gases from the cylinder and filling it with a fresh air/fuel charge is called scavenging. Two stroke engines use 2 different scavenging methods, cross-scavenging and loop scavenging. Both differing designs have particular advantages.

TWO STROKE CROSS-FLOW
two stroke cross-scavenged engines can be identified by the irregular shape of the top of the piston called a deflector. This deflector directs the incoming air/fuel up, towards the top of the cylinder. This creates a wall or column of fresh mix that sweeps across the cylinder towards the exhaust ports. As the column advances it pushes the spent exhaust gases out of the exhaust ports.
See Picture Below Hope this makes since to all and helps all.

http://www.maxrules.com/graphics/theory/cross.jpg

TWO STROKE LOOPER
Pistons in loop scavenged engines are generally near flat. They do not rely on deflectors to aim the fuel/air mix, rather they have shaped intake ports and combustion chambers to control the scavenging of the cylinder. Several intake ports are aimed upwards and arranged such that their combined streams flow upward and then LOOP down toward the exhaust ports.

Cross-Flow engines are better performing at idle and low speed. All older motors of any horsepower are of this design. Until the late 60's it was not economical to try to produce this design in quantity at a reasonable cost.

Looper engines, although having poorer idling characteristics are more fuel efficient and perform better at higher RPM's than crossflow as they have lighter pistons. This lowers the strain on the connecting rods, bearings and crankshaft. OMC created the first US production looper in 1968 with the 3 cylinder 55HP.

http://www.maxrules.com/graphics/theory/loop.jpg

[randlemanboater]

I don't have pic's, havent seen the boat in person yet, just trying to figure stuff out from afar.

Thanks Skoolie.

[Airslot]

Skools, great tutorial, thanks. Having on offline converstaion earlier, I realized that I don't understand exactly how the DFI's are lubing the bottom end. Care to shed any light on that?

Airslot

[Skools Out]

they have lower crank case oilers that are run from theon board oil pump and mixer.

[THEFERMANATOR]

LOOPERS tend to pull better up high and get slightly better milage. CROSSFLOWS pull good off idle but run out up high. OMC increased the bore on the loopers for 88 due to the lack of bottom end grunt(no replacement for displacement). OMC offered the 1.6L(crossflow) sea-drive all the way up to 89 that I've seen. 1.6L and 2.4L and 2.6L were crossflows, 1.8L 2L 2.7L 3L 3.6L and 4L SEA-DRIVES were loopers.

I personally can attest to the power potential that a small-bore looper can make though. The 19' COBRA bowrider in my sigg has run 53 MPH and was still climbing(but chine walking severely and no steering control). The same boat with a 150HP SUZUKI super six BASS pro edition ran 53 as well, and they were the same engine as a 225HP SUZUKI with a lower gear ratio foot.

[Skools Out]

well you can't base a hull on HP the hull design will only go so fast you could put twin 300's on a boat and it still run the same as the same hull with a single 250. all is based on hull design

[THEFERMANATOR]

well you can't base a hull on HP the hull design will only go so fast you could put twin 300's on a boat and it still run the same as the same hull with a single 250. all is based on hull design
Yep, but my hull with the 125 FORCE it origanally had would only pull off 35-38 with a light load on a good day. I can consistently pull off high 40's with my current prop with all sorts of loads(propped it down to keep from getting up to the uncontrollable speeds).

[Skools Out]

no i was saying all hulls have a top speed an once they reach their top speed you can add all you want but that's all the hull will go. now they may be faster than we want to go but they all have a top speed

[THEFERMANATOR]

I was just pointing out that these old loopers can RUN when they're right. Many people right off the JOHNNY-RUDES and say they're slow, but I've talked to many people and found that the small-bore loopers will get it, but there hard to get them right.

[Skools Out]

yeah you are better with a big bore looper same HP but were much better motors.

[THEFERMANATOR]

I fought my 85 for almost 2 years before I got it right. The previous owner gave up on it when he couldn't get it right. Ended up going with a set of early 87 carbs and running the timing at 19 degrees. Jetted the mains up 2 sizes and played with the idle and mid-range air bleeds to keep it from loading up and stalling on accel. It runs like a scalded dog now, but when I first got it it was a train wreck.

[randlemanboater]

Now there is some good info. Thanks guys for shedding light on all this.

Speaking of 1968 55hp omc's, thats what I learned to waterski behind when I was a kid. That motor would scream, 23 mph on a 14 ft Westwind!!!!

Ah the good old days.

[brianct12]

Randle,
I also learned to waterski ... pulled by a '69 era 55hp 3cyl with the electric, push-button shift. Brings back good memories. I seem to remember that motor (on an 18' fiberglass) pulling a skier at 25 mph.

[Airslot]

I learned to ski behind a 15' Cacicraft trihull with a 1970ish 60hp evinrude with the pushbutton electric shift. There was also an old wooden 15'er with a white merc, but I don't know any specs on the merc.

Airslot

[brianct12]

Hey Skools,
I'd enjoy learning more about a hulls top speed limitation. * I can understand there exists "a law of diminishing returns" ..... where you keep adding horsepower and not getting as much mph in return. * But I cant imagine a hull has a finite limit. *If I strap a 5000 lb thrust jet engine onto a V20 (assuming that was possible) ... its gotta move faster than one with a 250hp (or 2 250hp) outboards on it. *No ? *

[Skools Out]

lol yeah little different story there, at that point you most likely will join the Darwin Club lol. The coast guard buys 18ft Parkers with twin 300's on them to make you scared i guess cause the engineer said they are 3 mph slower than the same boat with a single 250 lol.

[bradford]

Hey Skools, What about drug smugglers who clamp on four yamaha 250's to the back of a boat that looks like it was built in someone's backyard?

[Airslot]

Skools in 98% correct and BrainCT is a touch right as well. Technically you are looking at "diminishing returns" as the horsepower continues to increase the gains become smaller and smaller. There is a line, where when reached, it takes huge amounts of additional power to see very small gains. That is the area reffered to as hull speed. On a V-20 per say, with a 150 you can get 43ish mph, add 50 hp and you add 4 mph. Put 500 hp on one and you'll likely get mid 60's in theory. If you jumped it to 600 you'd likely see little or no increase. The hydrodynamics of the last little bit of drag just can't be overcome, efficiently at least. That's the main reason that the big boat speed records only inch up. HP is easy to find and stuff into a 40'er. For them the HP isn't the limiting factor, the hull form is.

Airslot

[Skools Out]

hey remember that pic tin posted of the drug boats with 6 non counter 250's wow

[THEFERMANATOR]

hey remember that pic tin posted of the drug boats with 6 non counter 250's wow
There's one over on THT right now with 8 250's on the back.

[bigshrimpin]

http://www.historicraceboats.com/ph-mo108-400.jpg

Here's another . . . I thought max hull speed only applied to displacement hulls. I'm probably wrong but I know once you reach a certain speed . . . the power required to push the boat each additional MPH increases exponentially.

[bigshrimpin]

My dad had a 1969 johnson 55hp too on the back of a 17ft arrowglass. That motor had a green mid section a white cowling and roared like a nest of mad hornets.

[reelapeelin]

http://www.historicraceboats.com/ph-mo108-400.jpg

Here's another . . . I thought max hull speed only applied to displacement hulls. I'm probably wrong but I know once you reach a certain speed . . . the power required to push the boat each additional MPH increases exponentially.



Don't remember the ratio, but remember hearin' same thing about cars...once to 100 mph, takes 10 additional HP to increase 1MPH...or somethin' like that...


...an' I'm not sure I buy into a hull having a "Max Speed"...maybe max EFFICIENT speed...but let's say a given hull speed maxes out w/ 150 hp and you strap on a jet engine ... I think it's gonna out run the 150 ;) ...

[randlemanboater]

I know when I got my Starcraft/140 combo, I tried several different props. *I could make the holeshot better or worse, change the RPM at WOT and throughout the range, but the boat would go 43mph (gps) no matter what prop was used. *I can see why it wont go faster, with all those rivits sticking out of the bottom.

Now the guy that had the boat before me had a 150 Yamaha 2 stroke, he said he got 50 mph. But I dont know how he measured his speed.

[randlemanboater]

http://www.historicraceboats.com/ph-mo108-400.jpg



Fast or not, this boat scores a bunch of cool points, especially when this picture was taken, early 70's maybe?

The rest of us were being dragged around the lake by a Johnson 55 then.

[Blue_Runner]

I think every hull has a max speed. Its when the boat begins to break apart.

[randlemanboater]

TRUE!!!

I think the max speed is reached right before your boat becomes an airplane.

[Hammerhead]

Along with the V-20 I hava 1969 16' bonito walk-thru. It had a 65hp 3 banger johnson and cable steering when I bought it for 150 bucks. I replaced the deck( foam filled under it) repowered with a 115 evindude/ 21" SS prop. The boat was dangerous. At WOT it would chine walk. I still have the boat but, powered down to a 90 yam.

What's the fastest V-20s out there? The ICW limit is 25 around here and the ocean is always too rough to run WOT anyway.

[bigshrimpin]

What's the fastest V-20s out there? The ICW limit is 25 around here and the ocean is always too rough to run WOT anyway.

There was someone with twin 140's on the board . . . but I'll believe some of the early Alim v20 race boats were pushing 70+mph. CIA v20's with twin 100's were high 40's in 1961.



http://www.wellcraftv20.com/gallery/albums/album19/scan0002.sized.jpg

[bassarama]

Well looking at my motor owner's manual it says it's a loop charge. Are we talking the same here?

Also I was told by a pro that these "loopers" are equiped with low compression headers, thus running at lower compression.

Can someone translate that please? ;D

Thanks

Joe

[76GMC1500]

Low compression heads... *Using a low compression head reduces the fuel octane requirements of the motor. It also reduces fuel economy and horsepower.

I believe the compression ratio for a 2-stroke is actually the dynamic compression ratio. *A static compression ratio is the volume of the cylinder at BDC over the volume of the cylinder at TDC. *A 2-stroke dynamic compression ratio takes the volume of the cylinder when the exhaust port closes and divides it by the volume of the cylinder with the piston at TDC.

Lowering the compression ratio, raising the exhaust port in the cylinder so it closes later, increases the top end performance of a 2-stroke motor. *Lowering the exhaust port increases the bottom end. *Since the loopers are high rpm motors, they run low dynamic compression ratios.

[Skools Out]

bass the compression on Loopers have 120 to 130 usually on a V4 that's the same as a crossflow V4 but the V6 motors are different the loopers are 90 to 100 usually and crossflow's are 125 to 135. the Loopers have a single carb per cylinder. they are much better on fuel than crossflow due to the fuel flow inside the motor. look at the first page of the pics they show the fuel flow. Loopers produce less heat as well. if you have a 2 stroke choice go Looper for better fuel mileage, lower temps, better all around HP range, they turn more RPM's, and easier and cheaper to fix usually.

[THEFERMANATOR]

bass the compression on Loopers have 120 to 130 usually on a V4
Mine blows those numbers out of the water. I've got a 145 on one bank both cylinders, and 150 and 155 on the other side. Mine is a bare to start without dual batteries, or at least a good group 27.

[bassarama]

Those are very technical explanations, thanks. :)

Now, I really like my looper. ;D

[76GMC1500]

My 1cyl. looper makes 205 psi! Premium gas only for me.

[Skools Out]

is that on an outboard? that compression is too high for stock heads, crank, rods, and starter. i have a racing jet ski with all race mods stock was 90 PSI now 230 PSI. but once you get that high on a 2 stroke you have to weld the crank so the plates don't turn. You have to use better rods as well.

[Skools Out]

Mine blows those numbers out of the water. I've got a 145 on one bank both cylinders, and 150 and 155 on the other side. Mine is a bare to start without dual batteries, or at least a good group 27.


Does your motor run hot? More compression means more fuel use and higher temps.

[76GMC1500]

The 205 psi is in my 1987 CR 250 dirt bike, but it is a looper so I thought I would throw it in here.

[bigshrimpin]

I thought any two stroke above 145psi couldn't run on pump gas. That was the rule I heard . . . :P

[Skools Out]

I run VP12 114 Octane racing fuel in my jetski with the 230 PSI. I have run it on 93 Octane in her but doesn't like it much, only $5.99 a gallon for that puppy. That bike isn't a looper actually it's the same as my jetski they are called piston port motors. whole different world in 2 strokes.

[THEFERMANATOR]

Does your motor run hot? *More compression means more fuel use and higher temps.
It doesn't now, but the previous owner kept having issues with it(hence how I got it). It's got stock heads and flat top looper pistons, mine was an early one and came stock with the bigger rods. Mine was also before they lowered the compression for unleaded gas. They offer the low compression heads for it, but why change it. It runs great, I've got close to 200 hours on it since I did anything to it internal. And I even run it on 87 octane with no spark knock at load(but does rattle a little at idle on the low octane). It seems to get great economy to boot for a V4. 7-8.5GPH @ cruise and burns around 13GPH WOT.

[Skools Out]

hey that's not bad the highr compression will give it more power arrrr oh yeah more power

[THEFERMANATOR]

Yep, it runs pertty good. Like I posted earlier, it pulls off the same top end as a 150 SUZUKI on an identical hull. And there was a guy on the local lake none to happy when his 18' hydro stream with a 150 EVINRUDE XP couldn't catch me. It also didn't help that there was 5 in my boat and he was by himself in his ;D.

But those hydro streams can move, a buddy has an identical hull with an old built up 2.4L MERCURY said to putting out around 280HP. He's GPS'd it @ 98MPH before he ran out of lake with 2 on board. He figures he's had it up around 102 before, but no GPS at that time.

[handyman_special]

This is some great stuff u-all. What is the difference between a big bore and a small bore looper ? Also where do i look on my OMC to find the motor numbers.......ps also how many hp is 2.7 liter ?

[Skools Out]

the small bore has 3.500 pistons which were in all pre 1988 loopers and big bore have 3.685 bore in 1988 up looper 120,130,140,200,225,250,275,300 hp motors


V-4 Models
1.6-liter Crossflow, 1983-1987, and 1988-1990
1.8-liter Looper, 1985-1987
2.0-liter Looper, 1988-1990

V-6 Models
2.5-liter Crossflow, 1983-1990
2.6-liter Crossflow, 1983-1990
2.7-liter Looper, 1985-1987
3.0-liter Looper, 1988-1990

V-8 Models
3.6-liter Looper, 1985-1987
4.0-liter Looper, 1988-1990



the 2.7 was a small bore and has either 200 or 225 HP depends on what carbs you have and jets. do the carb butterfly's have 2 or 3 screws holding them in? 2 will be 200 Hp *carbs and 3 will be 225 Hp carbs.

[reelapeelin]

Again w/ the OMC Spec Wallpaper. ;D ... Skools, couldya just be a little more specific ;D ;D ;D ...

[THEFERMANATOR]

do the carb butterfly's have 2 or 3 screws holding them in? 2 will be 200 Hp *carbs and 3 will be 225 Hp carbs.
My 140(same carbs as a 225) only have 2 screws holding the butterflys in, and all of the 225's I've worked on have only had 2 screws holding the butterflys in. These were all 91 and older motors though. I know the throttle plates are different in between the two. They have different size air cuts in them, and are 2 different sizes. Except for 85. In 85 they kept the same carbs for both 120 and 140. The 120 had 57 main jets with 18 BTDC timing, and the 140 used 59 main jets with 22 BTDC timing. But OMC quickly found that this led to cylinder overheating(hence how I came abouts my 85 140).

[Skools Out]

My 140(same carbs as a 225) only have 2 screws holding the butterflys in, and all of the 225's I've worked on have only had 2 screws holding the butterflys in. These were all 91 and older motors though. I know the throttle plates are different in between the two. They have different size air cuts in them, and are 2 different sizes. Except for 85. In 85 they kept the same carbs for both 120 and 140. The 120 had 57 main jets with 18 BTDC timing, and the 140 used 59 main jets with 22 BTDC timing. But OMC quickly found that this led to cylinder overheating(hence how I came abouts my 85 140).


those aren't 140 nor 225 carbs those are 120 and 200 carbs. the 140 and 225 carbs all have 3 screws trust me i have 6 - 225's currently , 5 - 140's, 1 - 200, and 1 - 120

i have 3 extra sets of 200 carbs and 4 sets of 225 carbs. the 120 used smaller reeds and smaller carbs the 130 uses smaller reeds and larger carbs and the 140 uses the large reeds and large carbs.

[THEFERMANATOR]

My 140(same carbs as a 225) only have 2 screws holding the butterflys in, and all of the 225's I've worked on have only had 2 screws holding the butterflys in. These were all 91 and older motors though. I know the throttle plates are different in between the two. They have different size air cuts in them, and are 2 different sizes. Except for 85. In 85 they kept the same carbs for both 120 and 140. The 120 had 57 main jets with 18 BTDC timing, and the 140 used 59 main jets with 22 BTDC timing. But OMC quickly found that this led to cylinder overheating(hence how I came abouts my 85 140).
Disreguard most of this statement :-[. I went and re-checked my current 140 carbs and they do have 3 screws in them. But I'm 99.9% certain(can't be 100%, I threw them out when I started my last engine build needed space) that my 85 carbs had 2 screws in them. I know they were not 120 carbs because I compared them to a set of 120's before I got my 87 140 carbs. They had different holes drilled in them than the 120's had, but they did use pretty much the same butterfly size as the 120's.


Let the beatings begin.

[Skools Out]

lol it's ok here's a few of my spares

these are 200 carbs.

http://i8.photobucket.com/albums/a46/Skools_Out/Motors/MVC-049S.jpg

http://i8.photobucket.com/albums/a46/Skools_Out/Motors/MVC-055S.jpg

http://i8.photobucket.com/albums/a46/Skools_Out/Motors/MVC-050S.jpg

these are the 225 carbs

http://i8.photobucket.com/albums/a46/Skools_Out/Motors/MVC-051S.jpg

http://i8.photobucket.com/albums/a46/Skools_Out/Motors/MVC-053S.jpg


notice how much larger the throats are on the 225 carbs they are hugh compared to the 200's

[THEFERMANATOR]

My 87 carbs look like your 225's, but my 85 carbs' butterfly's looked NOTHING like either of them. I remember them having smaller throats than my current ones, but they had a big cut on them on the edge of the butterfly and only 1 big holed drilled in them. I had nothing but problems out of the 85 carbs with high idle. I could get it to idle at around 5-600 in gear and it would race up to around 1500 in neutral. I kne they had some vacuum leaks in them as well, but the plastic wasn't warped.

Oh well. Switched out to my 87 carbs, rejetted them and aint had no problems since.